There are many industrial and commercial uses for glass beads. For example, such beads have been used primarily to improve the reflectivity of a surface, such as on highway signs, in paint for pavement markings, on motion picture screens, and on advertising signs. Glass beads have also been used as fillers for thermoplastic and thermosetting resins. Other uses include the use of glass beads as the medium for grit blasting and for peening certain metals. Glass beads can also be used in reflective clothing, or they can be coated with metal and used as a conductive medium.
For many applications, a combination of properties is required for satisfactory performance. Typically, it is desired that the beads be as nearly spherical as possible, in order to provide optimal performance. Applications in which this is important include those in which reflectivity of the beads is crucial, for example in pavement marking, where the term “retroreflectivity” is used to describe the amount of light that reflects back in the direction of a source, typically headlights on a vehicle.
In pavement marking applications in particular, a number of other performance parameters are important as well, including resistance to degradation by the harsh mechanical and physical environment of a road surface. A high refractive index is needed for high retroreflectivity, but this is often at odds with one or both of the requirements for chemical and mechanical durability and the requirement for a high degree of sphericity. These properties are greatly influenced by the way in which the beads are made, by the particular glass formulations used in their preparation, and by interactions of these variables.
One method for forming highly spherical glass beads is disclosed in European patent EP 1,135,343B1 to Jackson et al., and employs either a rotating strike wheel or a jet of gas from a flame to break up a molten stream of glass into filaments. As these filaments pass through the heated region of the particular process, they form spheres. However, if the filaments do not reside in the heated region for a long enough time and/or at a high enough temperature, a high percentage of the filaments do not have time to form round beads. On the other hand, if the filaments reside in the heated zone for a period too long and/or at a temperature too high, then the beads tend to stick to one another and form satellite structures such as doublets or triplets, which are highly undesirable. According to the Jackson patent, the time and temperature conditions during which the filaments are maintained following their formation, prior to cooling and collecting the beads, are set according to an equation defining an optimal relaxation time during which a maximum percentage of the filaments reshape, or “spheridize”, themselves to produce spherical beads. During the relaxation time, the beads are in free flight through a heated relaxation zone. The relaxation time depends upon the particle diameter, the viscosity of the glass during the relaxation process, and the surface tension of the glass during that process.
Nonetheless, with respect to beads for pavement marking, there still exits a need to identify glass formulations having a refractive index high enough for pavement marking beads, and yet also providing sufficient hardness, crush resistance, and stability to chemical attack to be durable in pavement marking, without being susceptible to bubble inclusion and/or at least partial devitrification of the beads. Thus, it is not always possible to find conditions suitable for the formation of highly spherical glass beads having low levels of devitrification and low levels of gas bubble inclusion from glass formulations having a high refractive index and capable of providing high resistance to degradation by environmental exposure. In sum, there remains a need for processes and glass compositions capable of providing glass beads with high retroreflectivity and good resistance to chemical and physical degradation.